Process for determining wheel speed signal correction factors

ABSTRACT

The present invention discloses a method of determining correction factors by which the different tire-tread circumferences are ascertained and compensated, wherein a short-time correction factor (KZ Rn ) and a long-time correction factor (LZ RN ) is developed for each wheel, the deviation (D Rn ) of the short-time correction factor from the long-time correction factor is determined and the deviation is weighted by way of a band-pass filter (3), wherein subsequently a long-time average value (LM Rn ) of the weighted deviation (D&#39;Rn) is developed, and a partial value (TW Rn ) of the long-time average value (LM Rn ) is assessed, as a function of the driving condition, for the correction and adaption of the long-time correction factor (LZ Rn ).

The present invention relates to a method of determining correctionfactors by which the differences of the tire-tread circumferences areascertained in the evaluation of wheel speed signals, which are inputsignals for automotive vehicle control systems, wherein, in the absenceof control operations, the wheel speeds are measured, a reference speedis determined and the wheel speed is compared to the reference speed forproducing the correction factors.

International application WO 89/04783 discloses a process of theabove-mentioned type, wherein the wheel speed is measured during wheelslip free periods, correction values are determined from the deviationsof the wheels from this vehicle speed and, subsequently, the wheel speedof the individual vehicle wheels is corrected by using the correctionvalues.

All wheels are compared in pairs in this known process. The wheel pairhaving the lowest speed difference is identified, and the average speedof this wheel pair is assessed as the vehicle speed. An average value ofconsecutive measurements is then produced. Thereafter, a relation isestablished between the speed of the individual wheels and the averagevalue to determine the respective correction factor.

Difficulties are involved in thereby achieving the accuracy required foran anti-lock system (ABS) or a traction slip control system (TCS), forexample, and in additionally preventing false and misleading correctionvalues due to cornering, or the like.

German patent application No. 39 15 879 discloses a method, wherein abase speed is derived from the speed signals of the individual wheelsand a wheel factor or correction factor is developed for each wheelaccording to a predetermined algorithm. The correction factor multipliedwith the actual wheel speed results in the base speed. The wheel speedmultiplied with the correction factor, determined this way, is made thebasis of further signal processing instead of the actual wheel speed.The method is used to evaluate the wheel speed signals of a brake systemwith anti-lock or traction slip control.

Further, German patent application No. 40 36 742 discloses a circuitryfor a traction slip control system with brake management or enginemanagement which is used to improve the control function when a smallerspare wheel instead of a driven normal wheel is mounted. This circuitryis used to evaluate the rotational speed measured on the spare wheel bya correction factor measured when the wheel rotational behavior isstable. The rotational speed is then conformed to the measured speedvalue of the second driven wheel of the same axle so that the same slipcontrol threshold applies for the (smaller) spare wheel as for a normalwheel. In this case, the correction factor is also determined duringnormal travel and when the traction slip control system is notactivated.

An object of the present invention is to develop a method which involvesa particularly high degree of accuracy of adjustment of the differenttire-tread circumferences. The degree of accuracy should even renderpossible electronic control of brake force distribution on the basis ofthe wheel speed signals. Compared to an anti-lock system (ABS), theelectronic control of brake force distribution necessitates anadjustment of much greater accuracy, i.e. an adjustment ten times moreaccurate, of the tire-tread circumferences.

It has been found that this object can be achieved by a preferred aspectof the method of the present invention including that

a short-time correction factor and a long-time correction factor isdeveloped for each wheel (n),

the deviation of the short-time correction factor from the long-timecorrection factor is determined,

this deviation is weighted by way of a band-pass filter which weakensthe signals representative of the deviation below a bottom speedthreshold and above a top speed threshold,

a long-time average value of the weighted deviation is produced by wayof a low-pass filter, and

a partial value of the long-time average value is assessed, as afunction of the driving condition, for the correction or adaption of thelong-time correction factor.

Anti-lock control, traction slip control and the electronic control ofbrake force distribution is achieved by the high-precision correction ofthe wheel speeds according to the present invention. The variations ofthe wheel speed signals due to mounting of an emergency wheel,cornering, the influence of engine stall torques, air pressure losses orso-called rough road surfaces are compensated for. The resultingaccuracy is appropriate for the electronic control of brake forcedistribution. A calculation of the transverse acceleration is alsopossible.

In a preferred aspect of the present invention, the instantaneouslylowest wheel speed is taken as the reference speed, and the short-timecorrection factor of the respective wheel is developed by division ofthe reference speed by the wheel speed and by producing the averagevalue or filtering the quotient v_(min) /v_(Rn). To produce the averagevalue, appropriately, a digital filter or low-pass filter is used todevelop the short-time correction factor according to the relation##EQU1##

In the relation,

KZ_(Rn) is the average value of the short-time correction factor of thewheel n (n=1 to 4),

N is the number of loops or cycles, by way of which the average value isproduced,

V_(min) is the instantaneously lowest wheel speed,

V_(Rn) is the instantaneous speed of the wheel n,

C is a constant, for example, C=1000.

With a loop time in the order of 5 to 20 msecs, the average value isdeveloped over a period of time in the order of 50 to 200 msecsaccording to a preferred aspect of the present invention.

Further, the long-time average values of the weighted deviations in thepresent invention are produced according to the relation ##EQU2##

In the relation,

LM_(RN) is the long-time average value of the wheel n, is the averagevalue of the weighted difference (D'_(Rn)) between the short-timecorrection factor and the long-time correction factor of the wheel n,

M is the number of the summing intervals N.

Further, in one embodiment of the method of the present invention, thepartial values of the long-time average values are determined bycurve-weighting factors which must be determined by a diagonal, sidewiseand axlewise comparison of the long-time average values of theindividual wheels.

Finally, in another arrangement, the long-time correction factors arevaried as a function of the partial values and depending on themagnitude of the partial values.

Further features, advantages and possible applications of the presentinvention can be seen in the description of one embodiment, makingreference to the accompanying drawing which shows, in a symbolical view,the sequence of the method of the present invention, the individualsteps and functions.

In this diagram, the input quantity used for each wheel is a quotientv_(min) /v_(R1) which is produced by the instantaneously lowest speedv_(min) and the respective wheel speed v_(Rn), or V_(R1) if wheel No. 1is concerned.

A short-time correction factor KZ_(R1) is developed from the inputquantity v_(min) /v_(R1) by way of a low-pass filter 1. The filter inthe present case is a digital filter or digital low-pass filter 1 (TP1)which, in this example, calculates an average value of the input signalv_(min) /v_(R1) across 16 loops at a clock frequency of 7 msecs, i.e.,for a duration of 112 msecs. The output signal of the low-pass filter 1,i.e. the short-time correction KZ_(R1), is produced according to therelation ##EQU3##

When an average value over N=16 loops is produced and with a constantC=1000, depending on the signal resolution, a short-time factor of##EQU4## is achieved for wheel 1.

In this case, the short-time correction factor is always less than 1000,and the signal resolution is at 0.1%.

Subsequently, the short-time correction factor KZ_(R1) and the long-timecorrection factor LZ_(R1) of the respective wheel n=1 are compared in acomparator and differentiator 2, and the difference or deviation D_(R1)of the short-time correction factor from the long-time correction factorof this wheel is determined. The signal representative of the deviationD_(Rn) or D_(R1) is assessed by way of a band-pass filter 3 or weightedas a function of the vehicle speed. In a medium speed range which isbetween 60 km/h and 120 km/h in the present case, the band-pass filter 3allows the signal D_(R1) representative of the deviation to pass in fullextent, while a very great attenuation prevails below a bottom speedthreshold which is at 20 km/h and above a top speed threshold of 200km/h in the present example. In the embodiment shown, the signalattenuation decreases linearly in the range between 20 km/h and 60 km/hand increases linearly in the range between 120 km/h and 200 km/h. Ofcourse, other band-pass characteristic values are also appropriate,depending on the respective vehicle.

The weighted deviation D'_(Rn) or D'_(R1) is processed further in asecond digital low-pass filter 4 (TP2). A long-time average value(LM_(Rn) or LM_(R1)) is produced according to the relation ##EQU5##

In the present embodiment, a long-time average value LM_(Rn) is producedin intervals of approximately 14 seconds, i.e. 128×16 loops, meaning128×16×7 millisecs.

This way, the influencing variables of city traffic, such asacceleration maneuvers, narrow curves, etc., are filtered off in thelower speed range. The effects of high traction torques and engine stalltorques are reduced to the correction factors in the high speed range.

However, only a partial value TW_(Rn) or TW_(R1) of the long-timeaverage value LM_(Rn) or LM_(R1) is conducted and processed further inthe method of the present invention. The partial values are determinedas a function of curve weighting factors Gk which, in turn, are achievedby diagonal, sidewise and axlewise comparison of the long-time averagevalues of the individual wheels. In the predefined periods, i.e. every14 seconds, the long-time average values LM_(Rn) are compared, and acorresponding curve weighting factor Gk is calculated in the followingmanner:

a) diagonal comparison:

    |LM.sub.R1 +LM.sub.R3 -LM.sub.R2 -LM.sub.R4 |>=20(=2%)

Gk equals 1 is assumed in this case.

b) When condition a) is not satisfied, there is the assumption ofcircular-course driving or cornering. The long-time average values ofthe individual wheels are compared sidewise, and the curve weightingfactors in this case are defined according to the following pattern:

    |LM.sub.R1 +LM.sub.R4 -LM.sub.R2 -LM.sub.R3 |>10(=1%)Gk=2

    |LM.sub.R1 +LM.sub.R4 -LM.sub.R2 -LM.sub.R3 |>20(=2%)Gk=3

    |LM.sub.R1 +LM.sub.R4 -LM.sub.R2 -LM.sub.R3 |>30(=3%)Gk=4

    |LM.sub.R1 +LM.sub.R4 -LM.sub.R2 -LM.sub.R3 |>40(=4%)Gk=5

etc.

The index "1" refers to the left front wheel, and the index "2" refersto the right front wheel. The reference numerals "3" und "4" designatethe right and the left rear wheels.

c) The influence of engine stall torques may be identified by axlewisecomparison of the long-time average values. To this end, the long-timeaverage values are compared axlewise:

    |LM.sub.R1 +LM.sub.R2 -LM.sub.R3 -LM.sub.R4 |>10(=1%)Gk2

The curve weighting factor Gk is multiplied by 2 in this case.

Adopting the output signal of the attenuating circuit 5 is limited tothe long-time correction factor LZ_(Rn) or LZ_(R1) individually for eachwheel in a subsequent band-pass filter 6. To weaken the effect ofoverspinning of a wheel on the long-time correction factor, the partialvalue TW_(R1) =LM_(R1) /Gk (provided this value ranges between 0 and10%) is adopted in full. The partial value is limited to 10% when itranges between 10 and 20%, and the partial value is adopted 10% to zero% when it is between 20 and 30%. The course of curve indicative of theadoption is shown symbolically in the band-pass filter block 6.

The output signal of the band-pass filter 6 is evaluated in an adaptingcircuit 7 for the correction of the long-time correction factor LZ_(Rn)or LZ_(R1) which, finally, is returned from the output of the circuit 7to the comparator and differentiator 2.

The long-time correction factor is produced for each wheel in theabove-mentioned operation which applies to wheel 1 (n=1).

Finally, two switches are shown symbolically in the drawing. SW1 is openwhen disturbance of the input signals is detected.

SW2 is open when the long-time correction factor of the correspondingwheel is identified as "correct" and shall not be corrected for the timebeing. Switch SW2 is closed as soon as the filtered, averaged andweighted short-time correction factor, i.e. its partial value TW_(Rn)(TW_(R1) in the present case), exceeds a limit value. To this end, thepartial value TW_(R1) is ascertained by way of a counter 8 in theoperating cycle of the low-pass filter 4 (TP2), i.e. in the 14-secondcycle, in the present embodiment of the invention. The output signalsare counted. In the absence of need for correction, the value of theoutput signal of step 5 amounts to "0" or "1". These values are added inthe counter 8. When higher partial values TW_(R1) occur, the values arededucted from the contents of the counter 8. As soon as the contents ofthe counter do not reach a predetermined value, switch SW2 is closed.This way, the desired correction is performed very quickly in theinitial phase.

We claim:
 1. Method of determining correction factors by which thedifferences of a tire-tread cirumference are determined in theevaluation of wheel speed signals, which are input signals for anautomotive vehicle control system, wherein, in the absence of controloperations, the wheel speeds are measured, a reference speed isdetermined and the wheel speed is compared to the reference speed forproducing the correction factors, characterized in thata short-timecorrection factor KZ_(Rn) and a long-time correction factor LZ_(Rn) isdeveloped for each wheel n; n=1 to 4, the deviation D_(Rn) of theshort-time correction factor from the long-time correction factor isdetermined, this deviation D_(Rn) is weighted by way of a band-passfilter which weakens the signal representative of the deviation D_(Rn)below a bottom speed threshold and above a top speed threshold, along-time average value LM_(Rn) of the weighted deviation D_(Rn) isproduced by way of a low-pass filter, and a partial value TW_(Rn) of thelong-time average value LM_(Rn) is assessed, as a function of thedriving condition, for the correction or adaption of the long-timecorrection factor LZ_(Rn).
 2. Method as claimed in claim 1,characterized in that the wheel having the lowest wheel speed v_(min) istaken as the reference speed, and in that the short-time correctionfactor KZ_(Rn) of the respective wheel n is developed by division of thereference speed v_(min) by the wheel speed v_(Rn) and by producing theaverage value or filtering the quotient v_(min) /v_(Rn).
 3. Method asclaimed in claim 2, characterized in that, for producing the averagevalue, a digital filter or a low-pass filter is used which develops theshort-time ##EQU6## correction factor KZ_(Rn) according to the relationand in the relation, KZ_(Rn) is the short-time correction factor of thewheel n n=1 to 4,N is the number of loops or cycles, by way of which theaverage value is produced, v_(min) is the instantaneously lowest wheelspeed, v_(Rn) is the instantaneous speed of the wheel n, C is aconstant, for example, C=1000.
 4. Method as claimed in claim 3,characterized in that with a loop time in the order of 5 to 20 msecs,the average value is developed over a period of time in the order of 50to 200 msecs.
 5. Method as claimed in claim 1, characterized in that thelong-time average ##EQU7## values LM_(Rn) of the weighted deviationsD'_(Rn) are produced according to the relation and, in therelation,LM_(RN) is the long-time average value of the wheel n, is theaverage value of the weighted deviation D'_(Rn) of the short-timecorrection factor from the long-time correction factor of the wheel n, Mis the number of the summing intervals N.
 6. Method as claimed in claim1, characterized in that the partial values TW_(Rn) are determined bycurve-weighting factors Gk which are determined by a diagonal, sidewiseand axlewise comparison of the long-time average values LM_(Rn) of theindividual wheels.
 7. Method as claimed in claim 1, characterized inthat the long-time correction factors LZ_(Rn) are varied as a functionof the partial values TW_(Rn) and depending on the magnitude of thepartial values.
 8. Method of determining correction factors by which thedifferences of a tire-tread cirumference are determined in theevaluation of wheel speed signals, wherein the wheel speeds aremeasured, a reference speed is determined and the wheel speed iscompared to the reference speed for producing the correctionfactors,wherein a short-time correction factor KZ_(Rn) and a long-timecorrection factor LZ_(Rn) is developed for each wheel n, the deviationD_(Rn) of the short-time correction factor from the long-time correctionfactor is determined, this deviation D_(Rn) is weighted by way of afilter which speed-dependently weakens the signal representing thedeviation D_(Rn), a long-time average value LM_(Rn) of the weighteddeviation D_(Rn) is produced, and the long-time correction factorLZ_(Rn) is corrected dependent on the long-time average value LM_(Rn).9. The method of claim 8 wherein the deviation D_(Rn) of the short-timecorrection factor from the long-time correction factor is weighted byway of a band-pass filter which weakens the signal representative of thedeviation D_(Rn) below a bottom speed threshold and above a top speedthreshold.
 10. The method of claim 8 wherein the long-time average valueLM_(Rn) of the weighted deviation D_(Rn) is produced by way of alow-pass filter.
 11. Method of determining correction factors by whichthe differences of a tire-tread cirumference of tires of a vehicle aredetermined in the evaluation of wheel speed signals, wherein the wheelspeeds are measured, a reference speed is determined and the wheel speedis compared to the reference speed for producing the correctionfactors,wherein a short-time correction factor KZ_(Rn) and a long-timecorrection factor LZ_(Rn) is developed for each wheel n, the deviationD_(Rn) of the short-time correction factor from the long-time correctionfactor is determined, and the long-time correction factor LZ_(Rn) iscorrected dependent on a value derived by processing the deviationD_(rn) over a certain time period.